1. Field of the Invention
The present invention relates to composite particles for an electrode, a production process thereof and an electrochemical device.
2. Related Background Art
Known examples of cathode active materials of lithium ion secondary batteries include layered oxides (such as LiCoO2, LiNiO2 or LiNiMnCoO2), spinel structure compounds (such as LiMn2O4) and lithium-containing phosphates (such as LiFePO4).
Among these, although layered oxides allow the obtaining of high capacity (for example, 150 mAh/g or more), they have the problems of low thermal stability when highly charged and a lack of safety (overcharge stability). In addition, although the spinel compound LiMn2O4 (theoretical capacity: 148 mAh/g) has a stable structure and a high degree of safety (overcharge stability), it easily elutes Mn3+ ions at high temperatures (such as 45° C. and above), thereby resulting in the problem of low stability of battery properties at high temperatures accompanying anode deterioration caused thereby. Consequently, lithium-containing phosphates are used in place of layered oxides and spinel structure compounds from the viewpoints of safety and high-temperature stability. Examples of batteries using a lithium-containing phosphate for the cathode active material are described in Japanese Patent No. 3484003 and Japanese Patent Application Laid-open No. 2004-303527.
However, although a typical example of a lithium-containing phosphate in the form of LiFePO4 (theoretical capacity: 169 mAh/g) has a high degree of safety and high-temperature stability, the discharge voltage relative to lithium is 3.3 to 3.4 V, which is lower than that of other cathode active materials. In addition, these materials are extremely sensitive to the atmosphere (a reducing environment is required) and temperature conditions during synthesis, and are disadvantageous for inexpensive, large-scale production.
On the other hand, another lithium-containing phosphate in the form of LiVOPO4 (theoretical capacity: 159 mAh/g) has a stable structure and a discharge voltage roughly equal to other cathode active materials (3.8 to 3.9 V relative to lithium), but does not particularly require a reducing atmosphere during synthesis as with LiFePO4. However, it demonstrates the characteristic problem of lithium-containing phosphates of low electron conductivity, making it difficult to adequately demonstrate the properties thereof in electrode structures of the prior art in which it is simply mixed with a conductive auxiliary agent.